Field of the Invention
The present invention relates to an actuator driving device to be applied to an imaging apparatus or the like and a method for controlling the same.
Description of the Related Art
Conventionally, a positive and negative energization method under PWM (Pulse Width Modulation) control due to duty ratio-current characteristics with high linearity has been used as an actuator driving device. However, CMOS (complementary metal-oxide semiconductor) sensors have often been used as imaging elements in recent years, which may lead to the occurrence of phenomenon in which magnetic field noise caused by a change in current under PWM control adversely affects on electric charge information of an imaging element. The cause of the readily occurrence of noise when the positive and negative energization method under PWM control is employed lies in the repetition of energization in the forward direction and energization in the backward direction by adjusting the width of onpulse over a certain period of time. A change in driving current is large, and thus, a change in magnetic flux becomes large accordingly. In particular, a change in magnetic flux is maximized at the time of the rise and the fall of a driving voltage upon PWM driving, resulting in an increase in actuation voltage due to electromagnetic induction.
In order to prevent the phenomenon, Japanese Patent Laid-Open No. 2012-28841 discloses an imaging apparatus that includes an imaging element which photoelectrically converts an object image into electric charge information, an actuator which drives a correction lens for correcting hand shake, and a control unit which changes the driving frequency of the actuator at the timing for reading out electric charge information from the imaging element. When the current image taking mode is a mode in which magnetic field noise is produced by a driving device, the current image taking mode is set to another mode at a frequency in which no magnetic noise is generated. This can suppress the influence of noise caused by a change in magnetic field in response to the actuation of the actuator on electric charge information.
Japanese Patent Laid-Open No. 2011-221519 discloses an actuator driver that is switchable to output either an analog signal or a digital signal in response to the object condition at the time of shooting. The imaging apparatus includes an imaging element, at least one lens which forms an object image on the imaging element, and an actuator which drives at least one lens based on a control signal. A driver for outputting a control signal switches whether to output an analog signal or to output a digital signal in response to the object condition at the time of shooting. When the brightness of captured image is set to the condition, the driver outputs a digital signal, i.e., a PWM signal if the brightness at the time of shooting is equal to or greater than a predetermined level. At this time, even if the captured image is contaminated by noise, such noise gives only small adverse effect on the captured image, and thus, the captured image is less affected thereby. Power consumption at the time of driving the actuator can be saved as compared with the case where an analog signal is used. When the brightness at the time of shooting is less than a predetermined level, the driver switches output to an analog signal. In the case of analog driving, power consumption increases due to low power efficiency at the time of driving the coil, but the influence of magnetic field noise from the driving coil to the imaging element can be suppressed.
However, in general, a high driving frequency for the actuator for driving a hand shake correction lens may lead to switching loss, whereas a low driving frequency may cause an increase in current ripple, an appropriate frequency band exists for the actuator. Thus, in the method for changing a driving frequency in response to the image taking mode as disclosed in Japanese Patent Laid-Open No. 2012-28841, the motor characteristics are not constant. In order to stabilize the motor characteristics, the driving frequency of the hand shake correction lens needs to be constant, but the method has the disadvantage of maintaining the driving frequency at a constant level.
In Japanese Patent Laid-Open No. 2011-221519, a change in current is suppressed by using a continuously changing analog signal, resulting in a reduction in the influence of magnetic noise. However, Japanese Patent Laid-Open No. 2011-221519 does not disclose a solution when a digital signal, i.e., a PWM signal is used, and thus, a fundamental solution is not yet attained thereby due to high power consumption.